The generation of power by the conversion to useful work of potential energy represented by the difference in temperatures between surface water and water at some depth below the surface is known. Typically, the operation is carried out using surface water having a temperature of at least about 21.degree. C. and more likely a temperature of about 30.degree. C. or perhaps higher, while the water taken at some depth, for example, 2,000 feet or more below the surface will have a temperature of no more than about 10.degree. C. These temperature differences in waters at different levels generally will be found in tropical and sub-tropical ocean regions.
Systems for the generation of power using water taken from different depths and having different temperatures have been characterized as ocean thermal energy conversion (OTEC) systems. The OTEC systems are known to include a closed cycle thermodynamic system having a working fluid that is evaporated following heat interchange between the warm surface water taken from an adjacent ocean region, expanded through a turbine to a lower pressure and, thereafter, condensed by the transfer of its latent heat of evaporation to the cold ocean water pumped from a lower ocean depth. It has been suggested that the working fluid may include propane, ethane, ammonia or n-hexane to name a few. These working fluids all have boiling points suitably lower than that of water at the same pressure.
Typical of the prior art which discloses a system of the aforementioned type are U.S. Pat. Nos. 4,055,145 (Mager et al) and 4,450,689 (Moe). Another prior art system of somewhat similar nature is U.S. Pat. No. 4,087,975 (Owens).
U.S. Pat. No. 4,050,262 (Nakanishi) discloses a system for the generation of power that uses discharge water, previously used as a coolant in a nuclear reactor in place of warm surface water in heat exchange with a working fluid to evaporate the working fluid. As previously discussed, the working fluid then is expanded within the closed system, and cold ocean water is used to condense the evaporated, expanded working fluid by transfer of the latent heat of evaporation.
The OTEC Systems also include an open cycle energy conversion system which utilizes warm ocean water as a working fluid. To this end, the warm ocean water is introduced into a low pressure evaporator within which a small portion of the water flashes into water vapor. The water vapor is expanded through a turbine to a lower pressure and, then condensed by cold ocean water. Another form of open cycle energy conversion system may be characterized as a mist generating system which produces a water head that can be used to drive a hydro-electric generator.
In order to derive the cold ocean water, the closed cycle systems of the prior art use a pipe which communicates at one end within a heat exchanger of the closed cycle system and extends to an open end at the proper depth for entry of water. As indicated, the depth may be about 2,000 feet or more and the pipe may extend off shore along the shore line and ocean floor, through a distance of about one mile, or so. Both OTEC Systems use a pipe which may be a polyethylene tube.
The water that shall enter the pipe is pumped to the surface for use with the land based power generation facility along the shoreline. In the event that the power generation facility is a water based facility having its situs in the water, as a floating structure, then the pipe which communicates with the power generation facility will extend downward through the ocean water to the proper depth.
The prior art has recognized that there may be problems attendant to the use of pipes disposed as previously discussed. These problems may have their genesis in an exposure of the pipe to forces induced by unfavorable wave, current and wind conditions. In addition, the pipe may create a hazard in navigable waters. Further still, the size of pipe to provide the water required may be enormous, to render costs of material and operation prohibitive. In an article appearing in the Honolulu Star Bulletin, dated May 8, 1986, Section B, page 6, it was stated "[T]he plant (referring to a 100-megawatt plant) would also call for (sic) cold water intake pipe 80 feet in diameter, running 8,000 feet offshore and 3,000 feet down."
According to the prior art, the problems induced by unfavorable wave, current and wind conditions may be overcome by strengthening the connection of the pipe to the floating structure to withstand conditions of stress. See, for example, U.S. Pat. No. 4,312,288 (Finsterwalder et al). The problem of exposure of the pipe installed from the shoreline, along the ocean floor, is also addressed by Moe. According to Moe, the adverse effects on pipes which communicate ocean water from a depth, or for that matter from the surface of the ocean, to provide a source of warm and cold water to respective heat exchangers in a closed cycle thermodynamic system may be overcome by locating each heat exchanger in a separate cavity of the closed system and communicating the inflows and outflows of water between the heat exchangers and the water source. The cavities, according to Moe, are located in a rock area along the ocean floor, adjacent to the water source having sufficient depth to provide the required temperature differential, and the pipes are disposed so that their inlets and outlets extend through the rock area slightly into the water source at appropriate locations along the rock area.